There are many situations in which downhole tools must be selectively actuated. For instance, during hydraulic fracturing of a multiple zone well, one or more tools are provided at each zone, and each tool needs to be actuated so that fluid is diverted to flow outwards to fracture the surrounding formation. It is often desirable for the actuation to be performed in a sequential manner to allow the formation to be progressively fractured along the length of the bore, without leaking fracture fluid out through previously fractured regions.
The most common approach to tool actuation is still fully mechanical. Typically, balls of ever increasing size are dropped down the well bore. The balls pass though the first and intermediate tools, which have a valve seat larger than the ball, until they reach a tool in the well with an appropriate size of valve seat. The ball then seats at the tool to block the main passage and cause transverse ports to open thus diverting the fluid flow. However, the use of ever increasing balls requires ever decreasing seats, and in some cases the smaller seats may define significant flow restrictions, which is undesirable.
WO 2011/117601 and WO 2011/117602 each describe an improved system which uses balls of a substantially similar size and a mechanical counting device associated with each tool. Each dropped ball causes the mechanical counting device to linearly progress along the main bore in a predetermined number of discrete steps until reaching an actuation site of the tool whereupon the tool is actuated. The mechanical counting device can be located at an appropriate position (number of steps from the actuation site) for each tool such that the downhole tools are sequentially actuatable. This system has been found to be highly effective.
In the oil and gas industry there is a significant drive to improve the effectiveness and reliability of tools which are deployed and operated in a downhole environment. This is to ensure that the tools operate at maximum efficiency, have minimum risk of failure or imprecise operation, can be flexible according to operator requirements, and minimise any necessary remedial action, associated time delays and costs.